QUOTE(xfrendick @ Mar 14 2013, 02:00)
Clearly, it's with regards to the debate a while back regarding pony toilet habits. Twilight is pointing out that they lack anuses and, presumably, urethras, indicating that they, as a species, utilize the entirety of their food intake for energy, producing no waste.
Not necessarily, maybe they cry a lot. Or sweat a lot. Or... ok i'll stop there.